A number of digital systems transmit a signal comprising linearly added quadrature-related carriers which have been modulated with digital data channels. Each of the quadrature-related carriers has the same frequency. A variety of different modulation schemes, such as phase shift keying (PSK) and quadrature amplitude modulation (QAM), are often used. Such modulation schemes can be represented by a two-dimensional signal space diagram having four quadrants. Within this diagram, a data point, comprising a portion of data from each data channel, is plotted for each of the transmitted data combinations.
In the receiver, the incoming signal is demodulated using local carriers and the digital data channels are regenerated. These local carriers must be synchronous with the incoming carriers for coherent demodulation. Since the incoming carriers are often suppressed, the information necessary to control the phase of the local carriers must be derived from the received signal. The circuitry which derives the phase aligning or phase error signal is called a carrier recovery circuit.
Carrier recovery circuits can be categorized depending on how the phase error signal is derived. One category includes those circuits which derive a reference signal at the carrier frequency, or some harmonic thereof, by nonlinear processing of the received signal before demodulation. These circuits usually do not meet the low phase jitter objectives required in many system applications. A second category of carrier recovery circuits, known as baseband carrier recovery circuits, drives a phase error signal by processing the received signal after demodulation. While the second category of circuits reduces the phase jitter, the processing is generally limited to a particular type of demodulation. It would, therefore, be desirable to develop a carrier recovery circuit which provides low jitter and is adaptable for use in a number of different modulation schemes.